Level instrumentation provides fluid level measurements for a variety of purposes and in diverse fields. To name a few applications, oil and gasoline tank farms, waste water treatment facilities, and chemical plants utilize level instrumentation to determine the amount of material present in tanks and other vessels in connection with material storage and processing operations.
In a common type of level instrument, a float rides on the surface of the material in a tank and conveys motion to an optical encoder in a level transmitter system that is mounted to outside of the tank. Movement of the float stemming from change in the tank's fluid level drives rotation of an optical encoder disk within the encoder. The encoder disk is typically radially segmented with optically transparent regions spaced between opaque regions. A set of light emitting diodes (“LEDs”) arranged to face one side of the encoder disk emit light towards a set of aligned photoreceptors mounted to face the opposite side of the disk. Dependent upon the rotational position of the encoder disk, light from each LED either transmits through one of the transparent regions for reception by an aligned photoreceptor or is blocked by an opaque region. The encoder generates a code based on the pattern of photoreceptors that are in the binary state of receiving transmitted light verses in the opposite binary state of receiving little or no light. Since rotation of the encoder disk is coupled to the tank's fluid level, this code carries information describing a tank's fluid level.
One disadvantage of such conventional encoder systems is their susceptibility to age and environmental effects. The optical transparency of the encoder disk, the light intensity output by the LEDs, the responsiveness of the photoreceptors, and the performance of the associated mechanical and electrical components can each degrade with time and usage. Such degradation can decrease the encoder's ability to discern disk position and can result in unacceptable performance.
Conventional level transmitters typically use exactly one encoding technique, either absolute encoding or incremental encoding, to determine tank level according to disk position. Absolute encoding determines fluid level according to the absolute position of one or more encoder disks. The level sensing system maps each absolute bit code produced by the encoder to a resolvable tank level, in one-to-one correspondence. Conventional transmitters employing absolute encoding systems often struggle to track rapid changes in fluid level and thus may be prone to unacceptable performance in certain application conditions.
While incremental encoders are usually more responsive and thus may track rapid changes in fluid level, they too have disadvantages. Incremental encoding determines tank level based on encoder disk rotation as determined by counting encoder pulses. Using a known tank level that is typically acquired manually as a starting reference point, the level measurement system computes subsequent tank level measurements by monitoring change in level and accumulating the change to that starting-point level. Incremental-based measurements are typically susceptible to accumulated error that can result from power interruption or failing to detect one or more pulses.
Thus, a heretofore unmet need exists in the industry to address the aforementioned deficiencies and inadequacies.